For 40 million years, the plains of South America
have been ruled by an awesome group of predators...
the aptly named Terror Birds.
But recently all this has changed.
A new breed of killer has arrived,
one fierce enough to terrorise the Terror Birds.
Smilodon, the fabled sabre tooth.
The most powerful big cat of all time.
There is no such thing as a sabre tooth tiger.
The correct name for these beasts is sabre tooth cats,
and there have been many different species.
The largest, at over 300 kilograms, is this one -
Smilodon, which means knife tooth.
Originally from North America,
they came to South America two million years ago.
The balance of life here changed as they became the top predators.
They live in tightly-knit clans, and this is the story
of three dramatic months in the life of one particular group
led by a seven-year-old male called Half-Tooth.
Right from the beginning, Half-Tooth has problems.
Two rival males have appeared...
and they want Half-Tooth's crown.
Half-Tooth's territory
spans several hundred square kilometres of grassland
on one of the strangest continents on Earth.
Since splitting from Antarctica 30 million years ago,
South America has been isolated from the world
and as a result, its animals are unique.
Oddities thrive here like Doedicurus,
a spiky-tailed relative of the armadillo - the size of a car.
Even the large herds of herbivores are exotic -
these Macrauchenia are unrelated to any modern mammal.
And then, of course, there are the native predators -
the flightless Terror Birds.
These are three metres tall and called Phorusrhacos.
But the isolation of these animals came to an end
when the movement of the South American continent
caused it to collide with North America.
Across a land bridge
came new groups of mammals to colonize this exotic world.
Life here would never be the same again.
Cats were among those colonists.
No cat had ever set foot in South America before,
but now sabre tooths rule the grasslands.
Half-Tooth took over this clan three years ago,
breaking one sabre in the process.
The group is now made up of four adult females and their offspring.
All the youngsters are Half-Tooth's.
During his reign,
Half-Tooth has seen off many challenges from rival males...
but none as serious as this one.
Two rival males have wandered deep into Half-Tooth's territory.
As usual, he is quick to respond...
but these are brothers and they are working together.
Even by Smilodon standards,
Half-Tooth is a very large male,
but he has never before faced two opponents at once.
The odds this time are stacked against him.
The females read the signals and start to retreat -
their only concern is to protect their offspring.
These fights can be brutal and Half-Tooth has the most to lose -
not only control of the clan but possibly his life.
No match for the brothers, Half-Tooth is quick to back down.
He has at least escaped serious injury,
but his problems are just beginning.
Life for a lone male is a grim battle for survival.
Half-Tooth hasn't had to feed himself for three years
because it was the females in the clan that did the hunting.
Outcast from the rich grasslands now controlled by the brothers,
he heads into the scrub forest on the edges of his old territory,
where he must learn to scrape out a living.
With many of the creatures here he stands no chance.
Doedicurus, for instance, are not only well-armoured, they are huge.
These are ancient relatives of the armadillo,
yet they weigh almost 300 times as much.
Their spiked tail clubs alone can weigh 40 kilos -
as much as a cannonball.
Although fearsome defensive weapons,
the males mostly use their tails
to batter rivals into submission
during hormone-fuelled jousts for a mate.
The loser ends up with more than just his pride dented.
In the clan, tensions are running high.
The females will not accept the brothers
while they still have young.
Defeating Half-Tooth was only part of the battle.
If the brothers are to gain authority over the clan,
they'll have to kill the cubs.
Meanwhile, Half-Tooth has pushed deeper into the scrub forest
in search of food, but if anything, his task is getting more difficult.
The dense undergrowth harbours another colossus - a sloth -
but this is no ordinary sloth.
This is Megatherium -
a giant ground sloth.
It has huge claws,
is six metres from nose to tail,
and weighs more than an elephant;
a very different beast from its distant relatives, the gentle tree sloths.
Half-Tooth moves cautiously.
The sloth also boasts impressive defences.
Small bones beneath his skin act like chain mail.
Adults are virtually indestructible.
Half-Tooth wisely gives them a wide berth.
Weeks pass, and the wet season is in full swing.
The females have done their best,
but only two of Half-Tooth's cubs survive,
and the brothers are a constant danger.
They spend much of their time patrolling their new territory,
spraying the boundaries with scent as a warning to other males.
They will not attack the cubs while the mother is present,
but she must abandon them to go hunting.
This is when they are most at risk, and so this mother protects them
by hiding them in the dense undergrowth.
While the females are absent,
the brothers are spotted dangerously close to the hiding place.
The cubs' lives hang by a thread.
Now their only hope is to remain motionless.
Out on the plains, there are large herds of Macrauchenia.
Unique to South America, this species is the last of their kind.
They are prime targets for Smilodon.
Macrauchenia are fast, wary animals,
and getting close enough to kill one takes endless patience.
The females hunt together as a pack, inching forward to set the trap.
They have been closing in on this herd for two hours now.
Gradually, they fan out with the lead female moving wide -
trying to encircle their prey.
Smilodon are powerful killers and able to accelerate explosively,
but they are not built for long chases.
Unless they get close, they don't stand a chance.
One false move and they will have to start all over again.
The lead female drives the herd towards the other females.
Having chosen their target,
the cats close in.
Macrauchenia have outstanding manoeuvrability -
the victim jinks, completely wrong-footing her pursuer.
But that's why Smilodon hunt in groups.
A second cat brings her down.
Surprisingly, as the cats fight to restrain their prey,
they do not use their infamous teeth, in case they break them.
First they work together
to pin down the struggling animal under the weight of their bodies.
Only when the prey is totally subdued are the famous sabres drawn.
They are extraordinarily precise and accurate tools for killing.
Their teeth simultaneously sever the blood supply
and strangle the windpipe.
One bite and the prey is dead.
But these are the original double-edged swords.
The lethal 20-centimetre sabres are actually quite fragile.
They cannot bite into bone,
and so Smilodon can only eat the fleshier parts of a carcass.
This is very wasteful - something that hasn't gone unnoticed
by other carnivores on the grasslands.
The Terror Birds are supreme opportunists.
They may have been replaced as top predators,
but they have quickly found new ways of surviving, thanks to the cats.
The Smilodon have eaten as much as their sabres will allow them.
They leave the kill for the birds to pick over.
South America is not the only place Terror Birds are doing well.
Once the land bridge opened to North America,
they were one of the few creatures to spread north successfully.
While they eat these leftovers,
their cousins are running riot in Texas and Florida.
They only died out just before modern humans appeared.
After the hunt, the mother returns to where she hid the cubs,
as she has done several times over the last weeks.
She calls to them,
but her calls go unanswered.
Half-Tooth is soon spotted nearby,
roaming on the fringes of a territory that used to be his own.
But where he once sprayed,
he can clearly smell the scent of the brothers.
That is not the only signal they have left.
With the death of his cubs, his last tie with the group has been severed.
The takeover is complete.
Sightings of Half-Tooth have started to become rare,
and it is clear from his physical condition he is not thriving.
It is dawn, and Half-Tooth is exploiting his sensitive vision
to hunt in the low light.
Male Macrauchenia live separately from the herds
and often wander into the scrub forest to browse,
using their long noses to strip fresh leaves.
Half-Tooth waits and allows the Macrauchenia to wander ever closer.
There must be no mistakes this time.
But a giant ground sloth is about to ruin everything.
As he crashes through the undergrowth,
Half-Tooth's quarry starts moving away.
It's now or never.
Half-Tooth is too weak to follow.
The longer he goes without a kill,
the smaller his chance of making one.
Increasingly desperate,
he is seen still patrolling the same area later that morning.
He spots a Macrauchenia mother and young close to the forest edge.
They have wandered away from the main herd.
Maybe this time.
With a lightning strike of its massive beak and claws,
a Terror Bird comes from nowhere to kill the youngster.
But all is not lost for Half-Tooth.
He seizes his chance.
Scavenging is now his best hope.
But after finishing this kill,
Half-Tooth disappeared from this scrub forest.
Not far away, the brothers have completely usurped Half-Tooth.
With the last of the cubs dead, the clan has now accepted them.
The remaining adolescents are tolerated by the brothers
because they are female and will soon be old enough to mate.
And with no cubs to suckle, the older females come into heat again.
Killing Half-Tooth's cubs was not just a pointless act of violence.
With the cubs out of the way,
the clan can now raise the brothers' offspring
and pass their genes on to the next generation.
All in all, the clan is returning to normal.
The young females practise their hunting skills.
Although the Doedicurus they have picked on
has little time for their games.
The rainy season has come and gone,
and Half-Tooth has not been seen for weeks.
His old clan, however, is very much in evidence.
They have been making a kill every few days,
and today they're once again feeding well.
As usual, the Terror Birds are not far away,
but on this occasion there is another scavenger - a Megatherium.
They might be herbivores,
but they will occasionally go for carrion to supplement their diet,
and when they do, nothing gets in their way.
One of the brothers has been killed.
The rest of the Smilodon quickly abandon the kill to the giant.
In the blink of an eye,
the Megatherium has changed everything for the clan.
Now there's only one brother left.
Within days, Half-Tooth reappears.
He must have been closely monitoring his old territory.
So sensitive is the sense of smell in cats
that he can tell the identity and even the health of a male
who has left his scent.
In this case, the marks are easy to interpret -
where there were once two males, now there is only one.
Within the clan, things are going badly for the remaining male.
He was not the dominant of the two brothers,
and the females appear to have become unsettled again.
But it is not the females that the lone brother is worried about.
It is only a matter of time
before other males come looking for trouble.
He continues to patrol his territory,
but a challenge will come sooner than the brother expects.
Half-Tooth is back.
Encouraged by the signals,
he is wasting no time in trying to reclaim the clan.
A fresh scent trail leads him on.
Three months after losing to the brothers,
Half-Tooth is once again about to fight for this territory.
The odds this time have evened up.
The fight is brief and brutal, but Half-Tooth triumphs.
The brother is defeated...
and fatally injured.
Unlike Half-Tooth, this cat will get no second chance.
The short violent reign of the brothers is over.
A year later finds Half-Tooth surrounded
by new arrivals of his own.
He is king of the grasslands again.
Smilodon were very successful predators,
and survived up until as recently as 10,000 years ago,
but eventually the world around them changed too much.
The climate became cooler and drier.
The larger prey they specialized in killing disappeared.
The sabre tooth world
came to an end.
In our final episode, things will get much, much colder
as we visit our ancestors in an Ice Age, 30,000 years ago.
It was the toughest of times
and gave rise to one of the most awesome mammals of all -
the mammoths.